1. Field of Invention
The present invention relates to an optical transmission device in which a light emitting element and an optical fiber are coupled, and more particularly, to an optical transmission device that can achieve long-distance and high-speed optical transmission using a light emitting element having a light emitting surface that is parallel to a substrate surface.
2. Description of Related Art
Research has recently been conducted to replace conventional edge-emitting lasers with surface-emitting lasers as light emitting elements of optical transmission devices. Since the laser radiation angle of the surface-emitting lasers is small, even when laser light directly enters an optical fiber, not via a focusing lens, optical coupling can be performed with a relatively high efficiency.
A method for optically coupling a surface-emitting laser to an optical fiber without using a lens is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 10-148734. In the method of this publication, an optical-fiber fixing hole is formed in a block member, an optical fiber is positioned in the hole of the block member by being inserted therein from one aperture, and a surface-emitting laser is fixed to a surface of the block member at the side of the other aperture. In this method, however, since the positioning accuracy depends on the machining accuracy of the block member, high-precision positioning is impossible.
For this reason, when a surface-emitting laser is optically coupled to an optical fiber without using a lens, a multimode optical fiber (an optical fiber having a core diameter of more than 10 μm) has been used instead of a single-mode optical fiber having a small core diameter that needs to be precisely positioned (the core diameter is 10 μm or less).
On the other hand, in an optical transmission device using an optical fiber, a single-mode optical fiber is used for long-distance and high-speed optical transmission. It has been assumed that a multimode optical fiber cannot achieve long-distance and high-speed optical transmission because its transmission band is narrow.
In contrast, PCT Japanese Translation Patent Publication No. 11-505630 discloses that even a multimode optical fiber can perform optical transmission with a transmission characteristic similar to that of a single-mode optical fiber by making laser light from a surface-emitting laser enter the optical fiber so as to be offset from the optical axis (hereinafter “offset launch”).
In order to implement the offset launch, it is necessary to set the offset amount within a suitable range so as to increase the transmission band and to decrease the coupling loss. In a positioning method in which the amount of light emerging from an output end of the optical fiber is monitored (active alignment), it is impossible to detect a slight difference in offset amount. For this reason, in order to set the offset amount within the suitable range, it is necessary to precisely position a light emitting portion of the surface-emitting laser and an input end of the optical fiber.